Do not touch stamp files when running under build_server

Siso considers a target to be dirty if its output file's mtime is
different than what it stored in its database. This cl makes it so we do
not retouch stamp files when running a task under the build server.

Bug: 389714586
Change-Id: I8eb8543263ab04867647534a54c186d2afd446c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6171180
Auto-Submit: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1405719}